Bacteria often produce an external layer of polysaccharides, characterized by a definite primary structure, which in turn is responsible for sometimes remarkable physicochemical properties. Although the number of monosaccharides which constitute the polymers is rather low, the great number of different polysaccharides defined up to now shows the capacity of the microbes to exploit the possible isomers and linkage types of the building blocks. Furthermore, variability is often introduced by the presence of non-carbohydrate groups linked to hydroxyl, carboxyl or amine functions. In this chapter, examples of polysaccharides produced by Gram-negative and Gram-positive pathogenic bacteria are given, together with a description of those polymers that are interesting for industrial and biotechnological purposes. Some discussion is also devoted to the general features of shapes that polysaccharides may adopt in solution. The biological functions of these biomolecules are discussed particularly in relation to their role in human infection processes. The structures of the polysaccharides produced by species of the Burkholderia cepacia complex is reported as an example of a current investigation devoted to the understanding of the role of these biopolymers in lung infections.